Ambient light sensor with internal light cancellation

ABSTRACT

An electronic device includes a transparent surface, a light emitting device that emits light through the transparent surface, and a light sensor for receiving ambient light and providing an ambient light value. A retarder and a linear polarizer are placed between the transparent surface and the light emitting device. The retarder and linear polarizer may attenuate internal reflections from the transparent surface. The light sensor may have two channels and a second linear polarizer may attenuate the ambient light directed toward a second channel. A second retarder may be used with the second linear polarizer to attenuate the ambient light directed toward the second channel. A light detection circuit may use the difference between the two channels of the light sensor to provide the ambient light value.

BACKGROUND

1. Field

Embodiments of the invention relate to the field of ambient lightsensors; and more specifically, to ambient light sensors withcancellation of internal light sources.

2. Background

Electronic devices may include devices that emit light and devices thatsense light. For example, a device may provide a display screen, such asa light emitting diode (LED) panel, and an ambient light sensor thatdetects an ambient light level that in turn is used to control thebrightness of the display screen. It is necessary to avoid having lightfrom the devices that emit light fall on the devices that sense light toprovide accurate sensing. This may be done by physically separating thedevices that emit light from the devices that sense light. For example,the devices that emit light may be viewed through apertures in a devicehousing that are separate from other apertures in the device housingthat admit light to the devices that sense light.

Generally it is desirable to minimize the number of apertures in thedevice housing both to better seal the housing for the protection of thecontained components and to improve the aesthetics of the housing. Adisplay screen and other devices that emit light are sometimes mountedbelow a transparent surface that forms a surface of the device housing.It would be desirable to provide a structure that allows devices thatemit light and devices that sense light to be mounted below the sametransparent surface of the device housing while minimizing the effect oflight from the devices that emit light on the devices that sense lightto provide accurate sensing.

SUMMARY

An electronic device includes a transparent surface, a light emittingdevice that emits light through the transparent surface, and a lightsensor for receiving ambient light and providing an ambient light value.A retarder and a linear polarizer are placed between the transparentsurface and the light emitting device. The retarder and linear polarizermay attenuate internal reflections from the transparent surface. Thelight sensor may have two channels and a second linear polarizer mayattenuate the ambient light directed toward a second channel. A secondretarder may be used with the second linear polarizer to attenuate theambient light directed toward the second channel. A light detectioncircuit may use the difference between the two channels of the lightsensor to provide the ambient light value.

Other features and advantages of the present invention will be apparentfrom the accompanying drawings and from the detailed description thatfollows below.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention may best be understood by referring to the followingdescription and accompanying drawings that are used to illustrateembodiments of the invention by way of example and not limitation. Inthe drawings, in which like reference numerals indicate similarelements:

FIG. 1 is a pictorial view of an electronic device 100 that embodies theinvention.

FIG. 2 is a schematic representation of a cross-section of an electronicdevice that embodies the invention.

FIG. 3 is a schematic representation of a cross-section of anotherelectronic device that embodies the invention.

FIG. 4 is a schematic representation of a cross-section of yet anotherelectronic device that embodies the invention.

DETAILED DESCRIPTION

In the following description, numerous specific details are set forth.

However, it is understood that embodiments of the invention may bepracticed without these specific details. In other instances, well-knowncircuits, structures and techniques have not been shown in detail inorder not to obscure the understanding of this description.

In the following description, reference is made to the accompanyingdrawings, which illustrate several embodiments of the present invention.It is understood that other embodiments may be utilized, and mechanicalcompositional, structural, electrical, and operational changes may bemade without departing from the spirit and scope of the presentdisclosure. The following detailed description is not to be taken in alimiting sense, and the scope of the embodiments of the presentinvention is defined only by the claims of the issued patent.

The terminology used herein is for the purpose of describing particularembodiments only and is not intended to be limiting of the invention.Spatially relative terms, such as “beneath”, “below”, “lower”, “above”,“upper”, and the like may be used herein for ease of description todescribe one element's or feature's relationship to another element(s)or feature(s) as illustrated in the figures. It will be understood thatthe spatially relative terms are intended to encompass differentorientations of the device in use or operation in addition to theorientation depicted in the figures. For example, if the device in thefigures is turned over, elements described as “below” or “beneath” otherelements or features would then be oriented “above” the other elementsor features. Thus, the exemplary term “below” can encompass both anorientation of above and below. The device may be otherwise oriented(e.g., rotated 90 degrees or at other orientations) and the spatiallyrelative descriptors used herein interpreted accordingly.

As used herein, the singular forms “a”, “an”, and “the” are intended toinclude the plural forms as well, unless the context indicatesotherwise. It will be further understood that the terms “comprises”and/or “comprising” specify the presence of stated features, steps,operations, elements, and/or components, but do not preclude thepresence or addition of one or more other features, steps, operations,elements, components, and/or groups thereof.

FIG. 1 shows a pictorial view of an electronic device 100 that embodiesthe invention. The electronic device 100 includes a transparent surface102 that forms a wall of the housing 104 that encloses the device. Alight emitting device 106 is located in the housing 104 such that lightis emitted through the transparent surface 102. For example, the lightemitting device 106 may be a display screen that is viewable through thetransparent surface 102 as shown in the figure. A light sensor 108 isalso located in the housing 104 such that ambient light falling on atleast a portion of the transparent surface 102 is received by the lightsensor to provide an ambient light value responsive to the level ofambient light.

FIG. 2 shows a schematic representation of a cross-section of anelectronic device that embodies the invention. A retarder 200 and alinear polarizer 202 are placed between the transparent surface 102 andthe light emitting device 106. Ambient light 220 passes through theretarder 200 and the linear polarizer 202 after passing through thetransparent surface 102 to reach the light sensor 108. The retarder mayprovide a nominal quarter wave retardation to transform linearlypolarized light to circularly polarized light. The retarder may be abirefringent material such as a stretched polymer (PET, COP etc.) or abirefringent inorganic (Quartz, Sapphire, etc.).

In the embodiment shown in FIG. 2, the linear polarizer 202 is betweenthe retarder 200 and the light emitting device 106 such that the ambientlight 220 passes through the transparent surface 102, then the retarder200, and then the linear polarizer 202 before being received by thelight sensor 108.

In the embodiment shown in FIG. 2, the light sensor 108 is adjacent thelight emitting device 106. A photosensitive surface of the light sensor108 is at substantially the same distance from the transparent surface102 as a light emitting surface of the light emitting device 106, suchthat light emitted by the light emitting device is not directly incidenton the photosensitive surface of the light sensor. It will beappreciated that the light sensor may be located in other ways so thatlight emitted by the light emitting device is not directly incident onthe photosensitive surface of the light sensor.

Light 210 that is emitted by the light emitting device 106 and reflectedinternally by the transparent surface 102 is attenuated by the retarder200 and linear polarizer 202 so that the effect of the internallyreflected light 212 on the light sensor 108 is minimized. The emittedlight 210 first passes through the linear polarizer 202 and onlylinearly polarized emerges. The emerging light is polarized in a firstdirection as determined by the linear polarizer 202. The linearlypolarized light then passes through the retarder 200 and emerges ascircularly polarized light with a given handedness, for example righthand circularly polarized. The circularly polarized light then passesthrough the transparent surface 102.

However, depending on the angle of incidence with the transparentsurface 102, all or some portion of the incident circularly polarizedlight 210 may be internally reflected by the transparent surface 102.The reflected circularly polarized light 212 is circularly polarizedwith an opposite handedness from the incident circularly polarized light210, for example left hand circularly polarized. The reflectedcircularly polarized light 212 then passes through the retarder 200 andemerges as linearly polarized light. Because the reflected circularlypolarized light 212 has the opposite handedness from the incidentcircularly polarized light 210, the emerging linearly polarized light212 is polarized in a second direction that is orthogonal to the firstdirection of linear polarization 210. The emerging linearly polarizedlight 212 is therefore substantially attenuated by the linear polarizer202. Thus little of the light 210 that is emitted by the light emittingdevice 106 and reflected internally by the transparent surface 102reaches the light sensor 108. The attenuation of the emerging linearlypolarized light 212 depends on extinction ration of the polarizer, andbirefringence of the path that light travels. An extinction ratio of100:1 or higher is achievable.

FIG. 3 shows a schematic representation of a cross-section of anotherelectronic device that embodies the invention. A first linear polarizer202 is between the retarder 200 and the light emitting device 106 suchthat the ambient light 220 passes through the transparent surface 102,then the retarder 200, and then the first linear polarizer 202 beforebeing received by the light sensor 308. Light 210 that is emitted by thelight emitting device 106 and reflected internally by the transparentsurface 102 is attenuated before reaching the light sensor 308 asdescribed above for the embodiment shown in FIG. 2.

In the embodiment shown in FIG. 3, the light sensor 308 is located suchthat some light 310, 312 emitted by the light emitting device 106 isdirectly incident on the photosensitive surface of the light sensor. Thelight sensor has two channels 308, 309. Ambient light 220 and light 310emitted by the light emitting device 106 reaches the first channel 308of the light sensor to produce a first light level value.

The ambient light 220 is linearly polarized by the first linearpolarizer 202. A second linear polarizer 304 attenuates the ambientlight 220 directed toward a second channel 309 of the light sensorbecause the second linear polarizer is oriented orthogonally to thefirst linear polarizer 202. The amount of ambient light 220 that is notattenuated by the second linear polarizer 304 and which thereforereaches the second channel 309 of the light sensor is negligible. Thus,primarily light 312 emitted by the light emitting device 106 reaches thesecond channel 309 of the light sensor. The second channel 309 of thelight sensor produces a second light level value that is responsive tothe amount of light 312 emitted by the light emitting device 106.

The first light level value produced by the ambient light that falls onthe first channel 308 of the light sensor will be approximately doublethe second light level value produced by the ambient light that passesthrough the second linear polarizer 304 and then falls on the secondchannel 309 of the light sensor. The ratio of the first light levelvalue to the second light level value can be calibrated. The differencebetween the first light level value and the second light level value canbe used to remove the effect of the light 310, 312 emitted by the lightemitting device 106 when determining the ambient light level.

An ambient light detection circuit 330 is coupled to first channel 308and the second channel 309 of the light sensor. The ambient lightdetection circuit 330 provides an ambient light level value 332responsive to the difference between the first light level value and thesecond light level value. The ambient light level value 332 isindirectly responsive to amount of ambient light 220 reaching the firstchannel 308 of the light sensor because the effect of the light 312emitted by the light emitting device 106 is cancelled by taking thedifference between the two channels of the light sensor. The ambientlight level values may be analog voltages, digitally encoded values, orother representations of the ambient light level as may be used byambient light detection circuit 330 and/or the other circuits within theelectronic device 100.

FIG. 4 shows a schematic representation of a cross-section of yetanother electronic device that embodies the invention. A first retarder400 is between a first linear polarizer 402 and the light emittingdevice 106. The first retarder 400 may also provide additional benefitssuch as reducing the reflectance of the light emitting device's internallayers such as wires, and traces. Ambient light 220 passes through thefirst linear polarizer 402 and then the first retarder 400 before beingreceived by the light sensor 408.

In the embodiment shown in FIG. 4, the light sensor 408 is located suchthat some light 410, 412 emitted by the light emitting device 106 isdirectly incident on the photosensitive surface of the light sensor. Thelight sensor has two channels 408, 409. Ambient light 220 and light 410emitted by the light emitting device 106 reaches the first channel 408of the light sensor to produce a first light level value.

The ambient light 220 is linearly polarized by the first linearpolarizer 402 and then circularly polarized by the first retarder 400. Asecond retarder 440 is in the path of the ambient light 220 directed tothe second channel 409 of the light sensor. The second retarder 440 hasthe opposite handedness from the first retarder 400. The second retarder440 linearly polarizes the circularly polarized ambient light 220orthogonally to the direction of polarization by the first linearpolarizer 402.

A second linear polarizer 442 attenuates the ambient light 220 directedtoward the second channel 409 of the light sensor because the secondlinear polarizer is oriented orthogonally to the direction of polarizedlight emanating from the second retarder 440. Thus, primarily light 412emitted by the light emitting device 106 reaches the second channel 409of the light sensor. The amount of ambient light 220 that is notattenuated by the second linear polarizer 442 and which thereforereaches the second channel 409 of the light sensor is negligible. Thesecond channel 409 of the light sensor produces a second light levelvalue that is responsive to the amount of light 412 emitted by the lightemitting device 106.

An ambient light detection circuit 430 is coupled to first channel 408and the second channel 409 of the light sensor. The ambient lightdetection circuit 430 provides an ambient light level value 432responsive to the difference between the first light level value and thesecond light level value. The ambient light level value 432 isindirectly responsive to amount of ambient light 220 reaching the firstchannel 408 of the light sensor because the effect of the light 410emitted by the light emitting device 106 is cancelled by taking thedifference between the two channels of the light sensor adjusted for theattenuation of the light emitted by the light emitting device by thesecond retarder 440 and the second linear polarizer 442.

The portion of the first light level value produced by the light 410emitted by the light emitting device 106 that falls on the first channel408 of the light sensor will be approximately double the second lightlevel value produced by the light emitted by the light emitting devicethat passes through the second retarder 440 and the second linearpolarizer 442 and then falls on the second channel 409 of the lightsensor. The ratio of the first light level value to the second lightlevel value can be calibrated. The ambient light level values may beanalog voltages, digitally encoded values, or other representations ofthe ambient light level as may be used by ambient light detectioncircuit 430 and/or the other circuits within the electronic device 100.

While certain exemplary embodiments have been described and shown in theaccompanying drawings, it is to be understood that such embodiments aremerely illustrative of and not restrictive on the broad invention, andthat this invention is not limited to the specific constructions andarrangements shown and described, since various other modifications mayoccur to those of ordinary skill in the art. The description is thus tobe regarded as illustrative instead of limiting.

What is claimed is:
 1. An electronic device comprising: a transparentsurface; a light emitting device that emits light through thetransparent surface; a first retarder between the transparent surfaceand the light emitting device; a first linear polarizer between thetransparent surface and the light emitting device; a light sensor forreceiving ambient light that passes through the first retarder and thefirst linear polarizer after passing through the transparent surface. 2.The electronic device of claim 1 wherein the first linear polarizer isbetween the first retarder and the light emitting device such that theambient light passes through the transparent surface, then the firstretarder, and then the first linear polarizer before being received bythe light sensor.
 3. The electronic device of claim 2 wherein: the lightsensor is adjacent the light emitting device and a photosensitivesurface of the light sensor is at substantially the same distance fromthe transparent surface as a light emitting surface of the lightemitting device, such that light emitted by the light emitting device isnot directly incident on the photosensitive surface of the light sensor;and the electronic device further comprises an ambient light detectioncircuit coupled to the light sensor, the ambient light detection circuitproviding an ambient light level value responsive to light incident onthe photosensitive surface of the light sensor.
 4. The electronic deviceof claim 2 wherein: the light emitting device is between the transparentsurface and the light sensor; the light sensor includes a first channelreceiving light from both the light emitting device and the ambientlight, and providing a first light level value; and the light sensorincludes a second channel with a second linear polarizer oriented toattenuate the ambient light that has been polarized by the first linearpolarizer, the second channel receiving light from the light emittingdevice, and providing a second light level value.
 5. The electronicdevice of claim 4 further comprising an ambient light detection circuitcoupled to the light sensor, the ambient light detection circuitproviding an ambient light level value responsive to the differencebetween the first light level value and the second light level value. 6.The electronic device of claim 1 wherein the first retarder is betweenthe first linear polarizer and the light emitting device such that theambient light passes through the transparent surface, then the firstlinear polarizer, and then the first retarder before being received bythe light sensor.
 7. The electronic device of claim 6 wherein: the lightemitting device is between the transparent surface and the light sensor;the light sensor includes a first channel receiving light from both thelight emitting device and ambient light, and providing a first lightlevel value; and the light sensor includes a second channel with asecond retarder and a second linear polarizer oriented to attenuate theambient light that has been linearly polarized by the second retarder,the second channel receiving light from the light emitting device, andproviding a second light level value.
 8. The electronic device of claim7 further comprising an ambient light detection circuit coupled to thelight sensor, the ambient light detection circuit providing an ambientlight level value responsive to the difference between the first lightlevel value and the second light level value.
 9. A method for detectingan ambient light level falling on a transparent surface of an electronicdevice that includes a light emitting device that emits light throughthe transparent surface, the method comprising: placing a first retarderbetween the transparent surface and the light emitting device; placing afirst linear polarizer between the transparent surface and the lightemitting device; and receiving ambient light that falls on thetransparent surface and then passes through the first retarder and thefirst linear polarizer with a light sensor.
 10. The method of claim 9wherein the first linear polarizer is placed between the first retarderand the light emitting device such that the ambient light passes throughthe transparent surface, then the first retarder, and then the firstlinear polarizer before being received by the light sensor.
 11. Themethod of claim 10 further comprising providing an ambient light levelvalue responsive to light incident on the light sensor.
 12. The methodof claim 10 wherein the light emitting device is between the transparentsurface and the light sensor, the method further comprising: receivingthe ambient light with a first channel of the light sensor, the firstchannel also receiving light emitted by the light emitting device;receiving the light emitted by the light emitting device with a secondchannel of the light sensor; attenuating the ambient light directed tothe second channel with a second linear polarizer; and providing anambient light level value responsive to the difference between the firstchannel and the second channel.
 13. The method of claim 9 wherein thefirst retarder is between the first linear polarizer and the lightemitting device such that the ambient light passes through thetransparent surface, then the first linear polarizer, and then the firstretarder before being received by the light sensor, and the lightemitting device is between the transparent surface and the light sensor,the method further comprising: receiving the ambient light with a firstchannel of the light sensor, the first channel also receiving lightemitted by the light emitting device; receiving the light emitted by thelight emitting device with a second channel of the light sensor;attenuating the ambient light directed to the second channel with asecond retarder and a second linear polarizer; and providing an ambientlight level value responsive to the difference between the first channeland the second channel.
 14. Means for detecting an ambient light levelfalling on a transparent surface of an electronic device that includes ameans for emitting light through the transparent surface, the meanscomprising: first means for circularly polarizing light placed betweenthe transparent surface and the means for emitting light; first meansfor linearly polarizing light placed between the transparent surface andthe means for emitting light; and means for receiving ambient light thatfalls on the transparent surface and then passes through the first meansfor circularly polarizing light and the first linear polarizer.
 15. Themeans for detecting an ambient light level of claim 14 wherein the firstmeans for linearly polarizing light is placed between the first meansfor circularly polarizing light and the means for emitting light suchthat the ambient light passes through the transparent surface, then thefirst means for circularly polarizing light, and then the first meansfor linearly polarizing light before being received by the means forreceiving ambient light.
 16. The means for detecting the ambient lightlevel of claim 15 further comprising means for providing an ambientlight level value responsive to light incident on the means forreceiving ambient light.
 17. The means for detecting the ambient lightlevel of claim 15 wherein the means for emitting light is between thetransparent surface and the means for receiving ambient light, and themeans for receiving ambient light further comprises: means for receivingthe ambient light and also receiving light emitted by the means foremitting light; means for receiving only the light emitted by the lightemitting device; means for attenuating the ambient light directed to themeans for receiving only the light emitted by the light emitting device;and means for providing an ambient light level value responsive to thedifference between the means for receiving the ambient light and alsoreceiving light emitted by the means for emitting light and the meansfor receiving only the light emitted by the light emitting device. 18.The means for detecting the ambient light level of claim 14 wherein thefirst means for circularly polarizing light is between the first meansfor linearly polarizing light and the means for emitting light such thatthe ambient light passes through the transparent surface, then the firstmeans for linearly polarizing light, and then the first means forcircularly polarizing light before being received by the light sensor,the light emitting device is between the transparent surface and thelight sensor, and the means for receiving ambient light furthercomprises: receiving the ambient light with a first channel of the lightsensor, the first channel also receiving light emitted by the means foremitting light; receiving the light emitted by the means for emittinglight with a second channel of the light sensor; attenuating the ambientlight directed to the second channel with a second retarder and a secondlinear polarizer; and providing an ambient light level value responsiveto the difference between the first channel and the second channel.means for receiving the ambient light and also receiving light emittedby the means for emitting light; means for receiving only the lightemitted by the light emitting device; means for attenuating the ambientlight directed to the means for receiving only the light emitted by thelight emitting device; and means for providing an ambient light levelvalue responsive to the difference between the means for receiving theambient light and also receiving light emitted by the means for emittinglight and the means for receiving only the light emitted by the lightemitting device.